An in-mold expansion molded article is characterized by arbitrary shape, lightweight, heat insulating properties, etc. In particular, an in-mold expansion molded article obtained from polyolefin-based resin pre-expanded particles is superior to an in-mold expansion molded article obtained from polystyrene-based resin pre-expanded particles in chemical resistance, heat resistance, and a distortion restoration rate after compression. Because of these characteristics, the in-mold expansion molded article obtained from the polyolefin-based resin pre-expanded particles has been put to various uses such as an automotive interior material, an automotive bumper core, a heat insulating material, and a cushioning packaging material.
However, the expansion molded article made of a polyolefin-based resin generally has the disadvantage of being flammable, although it has the excellent properties as described above. In particular, the expansion molded article has high flammability and burns easily compared to a non-expansion molded article.
In recent years, there is a growing demand for automotive parts, building materials, and electrical and electronic products/components having flame resistance and self-extinguishing properties. To meet the demand, many studies have been conducted on the expansion molded articles with flame resistance.
Various methods for imparting flame resistance to the inherently flammable polyolefin-based resin are being studied, and a flame retardant is added in general. Various flame retardants such as a halogen-containing compound, a hydrated metal oxide, phosphoric esters, and a nitrogen-containing compound have been used for the polyolefin-based resin.
Patent Document 1 discloses adding a sterically hindered amine ether flame retardant (a non-halogen flame retardant) to polyolefin-based resin pre-expanded particles, thereby providing an in-mold expansion molded article that meets HF-1 according to the UL94 horizontal burning test for foamed materials without causing a problem such as the deterioration of the in-mold expansion moldability, as shown in Examples. Patent Document 2 discloses an in-mold expansion molded article that uses the same flame retardant as that disclosed in Patent Document 1 and shows self-extinguishing properties in the burning test based on FVVSS302. In general, the polyolefin-based resin in-mold expansion molded article having a higher density or a larger thickness burns more easily. Therefore, even with the use of the sterically hindered amine ether flame retardant, it is desired that the flame retardant performance be further improved.
Patent Document 3 discloses a flame retardant polypropylene fiber and a flame retardant polypropylene film that include 0.5 wt % or more of a phosphoric ester-based flame retardant and 0.4 wt % or more of an NOR type hindered amine-based stabilizer. Patent Document 4 discloses a flame retardant that includes a metal hydroxide, a hindered amine compound having a piperidine skeleton, and a phosphoric ester. However, the production processes of the polyolefin-based resin pre-expanded particles and the in-mold expansion molded article thereof involve, e.g., the impregnation of the resin particles with a blowing agent, heating, rapid pressure release, and heating with steam. Therefore, there is concern that decomposition of the flame retardant or a reaction due to the addition of the flame retardant may occur. Moreover, the polyolefin-based resin pre-expanded particles are filled into a mold, heated with steam or the like, and fused together to form an in-mold expansion molded article. In this case, if the moldability is deteriorated due to the properties of the pre-expanded particles, a commercial value of the molded article is significantly reduced. The additives mixed with the polyolefin-based resin can affect, e.g., the shape and size of cells, the interconnection between the cells, and the fusion of the pre-expanded particles, and thus such changes may result in the deterioration of the in-mold expansion moldability and the mechanical strength.
Patent Document 5 discloses flame retardant polyolefin-based resin pre-expanded particles that include a pentavalent phosphate compound containing halogen. However, the use of halogen-containing materials has been restricted in recent years. Moreover, a halogenated phosphoric ester differs from a non-halogenated phosphoric ester in degradability, compatibility with the resin, or the like. Further, Patent Document 5 discloses the addition of a flame retardant aid, but does not disclose the addition of the other flame retardants.